Lower extremity rehabilitation following joint reconstruction and replacement surgery focuses on increasing flexion, extension, and strength to restore mobility and quality of life. Many protocols describe the standard of care over the full course of rehabilitation. They include well-known manual therapy techniques and exercises. Upright and recumbent stationary exercise bikes from the fitness market are employed for advanced therapy exercise when the patients have achieved range of motion (ROM) sufficient to complete a pedal rotation, usually at a minimum of about 105-110 degrees (knee flexion angle). Patients work on increasing knee flexion to the target of 120 degrees active ROM and increase strength and endurance.
Cycling ergometers equipped with shorter pedal cranks than the commercially available fitness bikes have been shown to be effective in applying cycling exercise earlier in the rehabilitation process. While a set of discrete pedal arms that can be quickly changed out was tested, a single mid-length crank arm was adopted and recommended for clinical use. The extreme position is a pedal crank short enough for a patient to safely use within 24 hours of surgery. If it were fitted to an appropriately designed stationary bicycle, the system could be a replacement for the continuous passive motion (CPM) machine to provide passive, active-assisted, and active movement starting in acute-stage rehabilitation. The minimum length is about 2″ which allows about 60 degrees of knee flexion when the knee is in full extension.
The extension of the idea of a set of discrete, interchangeable pedal cranks is a pedal crank system where the distance from the axle to the pedal can be varied quickly, easily, reproducibly, and in coordination with seat adjustments in discrete steps over the therapeutically useful range of about 2″ to 6″. Variable-length pedal arm systems have been described only several times in the medical literature and offered commercially in several products. However, all such products are either lacking essential features that preclude their broad application or are inherently flawed. For example, the SciFit adjustable pedal and arm cranks vary only between 5″ and 7″, a range that is only marginally therapeutically useful because by the time a patient can complete rotations at 90-95 degrees knee flexion, progress is rapid and the adjustable cranks are not important. Further, neither the patients nor the therapists understand the quantitative relationship between pedal crank length and knee flexion angle, the OEM does not make such information available, and the crank positions are not marked.
Design options include fixing the pedal to the arm and moving the assembly or fixing the crank arm and moving the pedal along it. Further, standard design for locating the pedal or crank and securing it is based upon a retractable pin (captive or free) that is operated by the hand by fully withdrawing the pin from the holes in the pedal and crank arm (free) or by the hand or a digit such as the thumb actuating the captured pin by compressing the spring as the pin is withdrawn from its seat in a hole in the crank (captive). Withdrawing the pin allows the pedal to slide transversely along the fixed crank or the pedal crank arm to slide transversely through the crank arm's locking mechanism until a suitable length is found, at which point the pin is reinserted into the locking mechanism.
A massive system such as the SciFit locks the crank arm securely in place with two large diameter pins but cannot be sized to permit a 2-inch long crank. Smaller systems that may reach 2 inches in length use a single pin but the single pin and necessary machining tolerances cannot eliminate excess play, and subsequent vibration and small but perceptible regular movement as the force on the pedal varies during the pedal rotation. Depending on the design, the moving component can also bind as it slides along the fixed component during adjustment. The vibration and clicking annoy the patient and therapist but more importantly dramatically increases the wear of the sliding surfaces leading to premature component failure. A variable-length pedal crank assembly for use on a cycling ergometer in the orthopedic rehabilitation clinic or home must be rugged, quick and easy to adjust, simple and labeled. The crank or pedal must be fixed securely in place with no vibration at all adjustment points and simultaneously physically engaged to prevent the foot from loosening it under any circumstances and removing it from the system.